1. Field of the Invention
The invention relates to systems and methods for controlling gas discharge plasmas in sterilization systems that employ gas discharge plasmas.
2. Description of the Related Art
Plasmas produced using radio frequency (RF) generators in particular have proven to be valuable tools in processes for the sterilization of medical devices. For example, in U.S. Pat. Nos. 4,643,876 and 4,756,882, which are incorporated by reference herein, Jacobs, et al. disclose using hydrogen peroxide as a precursor in a low temperature sterilization system that employs RF plasma. The combination of hydrogen peroxide vapor and a RF plasma provides an efficient method of sterilizing medical devices without using or leaving highly toxic materials or forming toxic by-products. Similarly, Jacob, U.S. Pat. No. 5,302,343, and Griffiths, et al., U.S. Pat. No. 5,512,244, teach the use of RF plasmas in a sterilization process.
However, there are problems associated with the use of an RF plasma in a sterilization process. The RF plasma may leave residual hydrogen peroxide on the sterilized article. The residual amount of hydrogen peroxide remaining on the sterilized article depends upon the RF power applied to the article, the amount of time exposed to the RF plasma, and the material of the article. For example, while some plastics (e.g., polyurethane) absorb hydrogen peroxide, other materials (e.g., Teflon) absorb relatively little, thereby yielding less residual hydrogen peroxide after sterilization. in addition, inherent inefficiencies in the energy conversion from the low frequency (e.g., 60 Hz) line voltage to the RF (e.g., approximately 1 MHz-1 GHz) voltage; used to generate the RF plasma limit the power efficiency of such systems to typically less than 50%. Energy efficiency is further reduced by typically 5-20% by virtue of the losses from the required impedance matching network between the RF generator and the load. Such low energy efficiency significantly increases the cost per watt applied to the sterilized articles. The required instrumentation for using RF electrical energy (e.g., RF generator, impedance matching network, monitoring circuitry) is expensive, which also increases the cost per watt applied to the sterilized articles.
One aspect of the present invention is a sterilization system that comprises a low frequency power feedback control system for controllably adjusting a power applied to a low frequency plasma within a vacuum chamber of the sterilization system to remove gas or vapor species from the article. The power has a frequency of from 0 to approximately 200 kHz. The low frequency power feedback control system comprises a power monitor adapted to produce a first signal indicative of the power applied to the low frequency plasma within the vacuum chamber. The low frequency power feedback control system further comprises a power control module adapted to produce a second signal in response to the first signal from the power monitor, and a power controller adapted to adjust, in response to the second signal, the power applied to the low frequency plasma to maintain a substantially stable average power applied to the low frequency plasma while the article is being processed.
Another aspect of the present invention is a method of controllably adjusting a power applied to a low frequency plasma within a vacuum chamber of a sterilization system to remove gas or vapor species from the article. The power has a frequency of from 0 to approximately 200 kHz. The method comprises monitoring the power applied to the low frequency plasma within the vacuum chamber. The method further comprises generating a first signal indicative of the power applied to the low frequency plasma. The method further comprises adjusting the power applied to the low frequency plasma in response to the first signal to maintain a substantially stable average power applied to the low frequency plasma while the article is being processed.